1. Field of the Invention
This invention relates to improvements in control systems utilising working fluids, and in particular to pneumatic or hydraulic braking systems such as may be found on vehicles and in particular on semi trailers, trucks and the like.
2. The Prior Art
Australian patent application number 41104/89 discloses brake systems for prime movers and trailers and discuss problems associated with compressed air brake system design and, in particular, premature brake lining wear in a prime mover which results when predominantly gentle braking takes place. The earlier applications propose a system in which compressed air is fed into a brake control line which controls application of the trailer brakes in order to "prime" the trailer brakes system so that there is minimal delay in brake application after the brake pedal is depressed by a driver. The brake control line carries a signal from the foot pedal of the prime mover to a relay valve to activate the relay valve so pressurised fluid can flow from a reservoir which is supplied by a supply line to eventually activate a brake. The invention disclosed in the earlier applications has been found to significantly overcome problems associated with delay in application of the trailer brakes during gentle braking and therefore braking load is more evenly distributed between the brakes of the prime mover and the trailer thereby decreasing brake lining wear on the prime mover and also providing smoother and enhanced braking characteristics.
Since brake lining wear is associated with the prime mover rather than the trailer, and since prime movers and trailers are often owned by different organisations, it is of more concern to the owner of the prime mover to enhance braking characteristics of the trailer. The systems described in the above Australian patent application propose the injection of pressurised air in the control line at or near the relay valve associated with the trailer brakes. It would be advantageous to allow for injection into the brake control line at the location of the prime mover so that the pneumatic or hydraulic circuitry can be associated with the prime mover rather than the trailer and without the need for additional connection of pneumatic or hydraulic lines and electric cables above that which is normally required when connecting a trailer to a prime mover. Incorporation of the required hydraulic or pneumatic lines in the prime mover which would provide for the above mentioned priming of the trailer brake system would enable the prime mover to be used with conventional trailers and would also ensure that the owner of the prime mover has the advantage of equalising braking load during gentle braking and therefore reducing brake wear on the prime mover.